Python and C/C++ relationship
Using python you can import a module to use functionality inside that module. Many modules are written using python but a python module can also be a binary file similar to a shared library that was compiled from C/C++ source code. The python/C API reference at docs.python.org details how to write C code to do this.

Blender uses this C-API to provide modules that we can import from python scripts. The binary modules are merged into blender's binary file and the python interpreter built into blender takes care of giving us access to those binary modules. Most of the relevant source code can be found in source/blender/python.

So yes python is used to define blender's interface, the various functions and classes that we use may be implemented in python or C/C++, from the python scripts we run in blender we don't see any difference.